Variation on "asswipe" . To the best of my knowledge, coined in the mid-to-late 1970's by the late Al Ferrante in Rotterdam, NY: this is an homage to John Wayne's final film, "The Shootist", which was out in theatres at the time.

As Mr. Wayne's title character was so good with a gun, that he was considered a specialist, or like unto a scientist (hence "shootist"), a "wipist" is one whose skills would place them at the forefront of the asshole-cleaning industry!